reducing current by half

Discussion in 'The Projects Forum' started by GOBUCKSONE, Sep 23, 2013.

  1. GOBUCKSONE

    Thread Starter New Member

    Sep 23, 2013
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    I am working on a circuit that has the following inputs. 129 volts, 1.8 amps, 233 watts with 72 ohms resistance. I need to cut the amps down to aprox .85 amps. What are my options to accomplish this. I am limited in physical size, so can not simply add a resistor that is large enough to handle the wattage.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What is this 72 ohm load?
    Why must you cut the amps down?
     
  3. GOBUCKSONE

    Thread Starter New Member

    Sep 23, 2013
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    the 72 ohm load is a heater cartridge that is powered by the circuit. I need to reduce the current because the heater coils in the cartridge are ramping to quickly and causing failure. The normal amount of time needed to achieve optimum temp is aprox 7 minutes. By slowing the current flow, I should be able to increase the amount of time for this to happen.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Simply adding a resistor would also simply reduce the power you got to the heater coils when all was said and done, anyway.

    Is this a DC circuit?

    The approach that comes to mind is to use a small microcontroller and a power MOSFET or solid state relay and PWM the power to the cartridge. That would allow you to tailor the ramp rate to whatever you wanted.
     
  5. GOBUCKSONE

    Thread Starter New Member

    Sep 23, 2013
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    it is a modified AC circuit. We have added a diode at the input end of the circuit to reduce voltage from 240 to 120v. This has also had the effect of "Cleaning" the noise out of the standard AC circuit. This, I believe, is what is allowing the heater coils to get to temperature so rapidly. They are achieving temp in 3 minutes, where if I plug into a standard 120 volt outlet (with-out the diode) it takes 7 minutes to achieve temp.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    If it's an AC circuit I would add a step down transformer that matches your needs.
     
  7. WBahn

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    Can you use a variac?

    I'm assuming you aren't achieving the same end temperature as you would if the coils were plugged into 240VAC instead of a half-wave rectified 120VAC.

    Where does this 72 ohms of resistance come from? Are these resistive filaments, or inductive heaters, or what?
     
  8. t06afre

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    May 11, 2009
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    How about using a diode in series with proper rating. If you cut out one of the half periodes the average effect should drop to half
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Why don't you explain "in detail" what you are really trying to do as to get the best answer?
    I can't tell if the "time" is the most important thing here or "temp" or what..
     
  10. GOBUCKSONE

    Thread Starter New Member

    Sep 23, 2013
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    Thank you all for your input and suggestions. Let me give you some more information about this circuit and it's intended use. We are trying to enter an electric cattle dehorner into europe with out using a step down voltage convertor. We have been building 120 volt units for over 40 years. Due to the amount of heat needed to kill the horn cells on an animal (over 850 degrees F) at 240 volt AC, the coil density of the heater cartridge would be so tightly wound that the coils would short against each other. There is not anything on the market capable with in our size restraints. So we are trying to build an internal circuit on the power cord of the dehorner that will drop the europeen voltage down to 115v - 130v. We are able to achieve this with the diode, but now the effeceincy of the unit is burning up our heaters.
     
  11. WBahn

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    Mar 31, 2012
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    Your problem is that you are still dumping twice as much power into your coils as you want.

    Power goes as the voltage squared. So if you take a coil rated for 120W when run from 120VAC, you will dump 480W into it at 240VAC (assuming constant resistance filaments, which is not a particularly good assumption). By inserting the diode you cut that in half to 240W.

    Put a 2:1 stepdown transformer in your unit, as recommended by gerty.
     
  12. t06afre

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    Have you explored using other materials/alloys materials in the element
     
  13. Alec_t

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    Sep 17, 2013
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    If the heater is purely resistive a lamp-dimmer type circuit should do the trick.
     
  14. WBahn

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    It's already half-wave rectified (see Post #5). How will adding another diode in series materially change anything?
     
  15. WBahn

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    Aren't converters pretty commonplace in Europe? Can't you get an inexpensive converter and package it with the unit -- perhaps making it a wall-wart type arrangement?
     
  16. ErnieM

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    Apr 24, 2011
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    I hate the diode idea, and so will any power company

    With a simple resistive load you should be able to use a triac dimmer to reduce the drive voltage to a proper level... and it also gives your customer a very empowering knob to twiddle.

    For something fixed a transformer would be idea, 240 V in/ 60V out.
     
  17. killivolt

    Active Member

    Jan 10, 2010
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    I think the ROI is to give the customer a solution that is not, a Transformer. But, a Transformer is what the customer should use. The amperage and safety is compromised, by not doing so.

    The customer should understand and realize this and should pay accordingly. Although, they should already have their own solution in the UK.

    Probably the solution is rhetorical, and to state anything different will defy a reduction in price.

    Therefor, an electronic solution becomes the holy grail.

    kv
     
  18. wayneh

    Expert

    Sep 9, 2010
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    Any reasonable long run solution ultimately involves a redesign of the coils, not rigging some adapter. Maybe you only need a short run solution today, but any "conversion" approach adds cost and removes elegance. Customers will not be impressed and your competitors will eat you alive. Just my marketing 2¢. I wouldn't launch until the product was as perfect as I know how to make it.
     
  19. richard.cs

    Member

    Mar 3, 2012
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    The diode method does not reduce the rms voltage to half, it reduces to power by half. Your rms voltage after the diode will be about 170V not 120 and if you've designed the heating coils for 120V that will be why they burn up. If you must go with the diode you will need to redesign the heater but it's a pretty poor choice in nearly all respects.

    Phase angle control with a triac, similar to a conventional lamp dimmer, is a bit better and can be done cheaply and within your space constraints, but it might be better to PWM the mains with a single mosfet.
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    An SCR circuit like this should work. I've never built one so lets get some other input as to the design. :D
     
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